1 Field of the Invention
The invention relates to a line driver for a network of data stations and more particularly to such a network wherein control of the transmission between stations is shared by the respective stations.
2. Description of the Prior Art
Prior art terminal networks usually have been controlled by a master computer which receives transmission requests from the respective terminals and grants access to a transmission channel by the individual terminals when the channel is available and according to some priority arrangement. Such master computers add to the cost of the terminal network and are not required for some networks where the terminals need communicate only between themselves or with a common storage file. Thus, it is desirable to have a terminal network where the transmission control is imbedded in or shared by the stations making up that network.
Prior art networks not having a master controller have employed "contention" schemes whereby each node of the network contends for the transmission medium whenever it is ready to transmit. One of the earliest contention networks was the "Aloha" system built by the University of Hawaii. In this system, each node transmitted whenever it had a packet ready for transmission. Whenever a portion of one node's transmission overlapped with another node's transmission, that overlap destroyed both packets. If the sending node did not receive an acknowledgment within another packet from the destination node after an arbitrary time period, it would assume that a collision had occurred and retransmit. In order to avoid continuously repeated collisions, some method of introducing a random transmission delay had to be introduced.
An attempt at reducing the effects of collision in contention-type networks is disclosed in the Metcalfe et al U.S. Pat. No. 4,063,220 which is directed toward a communication network in which each node is capable of detecting when collision occurs on the channel during that node's transmission, and interrupts its transmission when such a collision occurs. A random number generator is employed to select an interval of time to delay before the next attempted transmission. However, the collision detection mechanism adds to the complexity of the respective nodes with an increase in the cost of the system. Another contention-type network which does not require collision detection mechanisms is disclosed in the Malcolm et al U.S. Pat. Application Ser. No. 145,606 filed May 1, 1980 wherein each station will not begin transmission until it determines that the channel is in an idle state and, once the station has determined the channel is idle, it will then delay for a period of time that is randomly chosen; and, if the channel is still idle, will then begin transmission. With this type of system, a relative synchronization between transmission cycles for the various stations occurs, thus minimizing collision. If collision does occur, it is detected by the receiving station as a data error and that station simply does not return an acknowledgment signal.
A particular problem with contention networks is that when two different stations attempt to drive the channel at the same time and are separated by a relatively large distance, the transmission of each station will dominate nearby stations, thereby preventing their receiving transmission from a distant station.
It is, then, an object of the present invention to provide an improved line driver circuit for different stations in a contention network.
It is another object of the present invention to provide an improved line driver circuit for a station in a contention network where each node or station contends for access to the channel medium in a manner such as to minimize conflicts between the respective stations.
It is still another object of the present invention to provide an improved line driver circuit for a station in a data communication network, which driver circuit will not dominate reception by neighboring stations of transmissions from a distant station.